Plastic materials have a tendency to accumulate static electrical charges due to low electrical conductivity. Friction between dissimilar electrical insulators can generate significant static charge in a short period of time (i.e., triboelectric charging). Floor surfaces, particularly in a manufacturing area, are subject to such charge generation by virtue of the movement of material and people handling equipment.
This static charge is undesirable for a variety of reasons, including dust attraction, interference with processing during fabrication of the final product, and spark generation from static buildup, which can produce serious accidents such as fire or explosion.
The increasing complexity and sensitivity of microelectronic devices make the control of static discharge of particular concern to the electronic industry, such as integrated circuit (IC) and semiconductor equipment manufacture. Even a low voltage discharge can destroy or cause a latent defect in an electronic component. These adverse effects are often seen when the electronic component is a solid state electronic device, such as a computer chip, which contains several semiconductive layers. Due to miniaturization and assignment of many functions to a single chip, the semiconductor layers are required to be very thin. Thus, even relatively small electrostatic discharges of less than 500 V can burn through the semiconductor layer and induce latent defects in, or completely destroy the functionality of, the semiconductor layer.
Keys to controlling electrostatic discharge (ESD) include the use of grounding equipment and/or the elimination of static-generative materials. The latest technology incorporates inherently static-dissipative and anti-static polymers. An advantage to using polymers for ESD protection is that they can be tailored to a desired range of surface and volume resistivity, so as to be able to “bleed off” or dissipate any occurring static charge. The resistivity must not be so low as to allow the charge to move too quickly through material, thereby causing an arc or spark. On the other hand, the resistivity must not be so great as to cause the charge to build up to such a high level as to ultimately cause a sudden discharge (spark or arc).
The need to control electrostatic charge buildup and dissipation often requires the entire electronic assembly environment to be constructed of partially conductive polymers. It also may require electrostatic protective packaging, tote boxes, holders, housings, casings, and covers be made from conductive polymeric materials to store, ship, protect, or support electrical devices and equipment. It is estimated that plastics used in electric devices will increase in both dollar volume and types of applications they serve. For instance, sales of these materials will grow from $251 million dollars in 1987 to an estimated $768 million in 2001. Poundage is expected to increase accordingly, from 250 million to 767 million pounds over the same period.
Dissipation of electric charge from polymer surfaces is often accomplished by the addition of various conductive chemicals, but these chemicals often have short life spans or toxic constituents. Another mode of imparting some electrostatic charge dissipation to polymers involves placing conductive filler particles or dopants (hereinafter, “additives”) into a continuous polymer phase by blending the particles with the host polymer as it is extruded or otherwise processed. These additives are disclosed in several patents, e.g., U.S. Pat. Nos. 4,288,352; 4,634,865; 4,804,582; 5,232,775; and 5,955,526. Such additives include tin, lead, silver, copper, gold, carbon powder or fibers, nickel coated carbon fibers, stainless steel fibers, and ceramic materials. However, these technologies have several manufacturing and performance limitations. For example, the levels of additive that are required to provide sufficient conductivity for dissipating the electrical charge are very high, as much as 30% by weight. While blending conductive additives such as graphite and metals with a host polymer can increase conductivity and produce a dissipative solution, the finished product can suffer from a reduction in physical strength and inconsistent performance due to non-uniform distribution. Another problem associated with conductive additives is migration. At high temperatures (e.g., during processing or in field conditions), conductive additives tend to migrate to the surface of the polymer composition, which has a negative impact on both physical characteristics and ESD characteristics.
One conductive additive that has been blended with a base polymer to impart electrostatic dissipative properties is ferrocene. As a member of the metallocene family, each ferrocene molecule is composed of a metal atom bounded by two cyclopentadienyl rings. In the case of ferrocene, the metal center is iron.
Polymeric materials which contain organometallic moieties have become a subdiscipline of polymer chemistry. The ferrocene unit, for example, has proven itself to be a versatile building block, possessing very useful properties including high thermal stability, radiation resistance, and electroconduction properties.
Ferrocene moieties have been incorporated as a constituent of polymers as pendant groups, connected to the polymer chain by one functionality, and as integral polymeric units, connected to the polymer chain by two functionalities. For example, segmented poly(ether urethane) films containing ferrocene units in their hard segments have been developed (Gonsalves et al. (I), 1986), as well as ferrocene-modified urethane block copolymers (Najafi-Mohajeri et al., 2000). Such ferrocene polymers have been used previously in a variety of applications, such as catalysts, flame retardants, and photosensitizers (Gonsalves et al. (II), 1984; Hale et al., 1989; Kishore et al., 1991; Wright et al. (I), 1992; Wright et al. (II), 1992; Abd-Alla et al., 1993; Casado et al., 1995; and Najafi-Mohajeri, 1999). However, the ESD behavior of polymers containing a ferrocene moiety as a constituent of the polymer structure (as opposed to its use as an additive) has never been evaluated.
Accordingly, there remains a need for polymer compositions that exhibit electrostatic dissipative properties without the disadvantages associated with conventional dissipative polymers, which utilize conductive additives.